Existing ultrasound transducers are mostly based on piezoelectric materials operating in the longitudinal length-extensional d33 mode. Ultrasound composites are commonly operated in a longitudinal length-extensional mode, with bars of identical dimensions embedded in an epoxy matrix and electrodes applied to the common surface formed by the ends of the bars. This design produces a single resonant frequency.
The predominant material in ultrasound composites employing piezoelectrics is lead zirconium titanate (PZT) ceramics, although alternative materials such as polyvinylidene difluoride (PVDF) are also used. To maximize performance, the PZT is formed into 1-3 composites, which achieve bandwidths of up to approximately 80%. Although PVDF provides bandwidths in excess of 100%, it does so at the expense of sensitivity. Furthermore, the electromechanical coupling of longitudinal length-extensional modes of the PZT and PVDF are generally less than 70% and 10%, respectively. These low coupling values do not make it advantageous to use the longitudinal length-extensional modes for sensitive, multi-octave bandwidth transducers.
In piezoelectric devices using the d33 mode, frequency is inversely proportional to piezoelectric thickness. Lower frequency transducers thus require thicker material. However, voltage requirements scale with material thickness, meaning that higher voltages are also required. As a result, low frequency transducers made with conventional technology are large and consume more power. High frequency transducers, on the other hand, require smaller feature sizes. These small feature sizes are difficult to produce with conventional processing techniques (i.e. dicing saw), but capacitance of the elements decreases and requires the system to be more complicated to achieve adequate sensitivity.
Accordingly, known transducers are generally limited to a bandwidth centered around a single resonance because a single transducer is generally not capable of spanning both high and low frequencies, particularly in small sizes. As a result, bandwidth is not adequate for certain end-use applications and multiple transducer probes may be necessary.
The limited bandwidth of current transducers centered around a single resonance affects acoustic diagnostic techniques. Performing measurements on a target material (e.g. biological tissue or a metal structure) may require different frequencies to identify different structural parameters. Using multiple probes to achieve measurements at differing bandwidths is unsatisfactory because it adds uncertainty whether the same spatial area is being evaluated.
Limited bandwidth also affects acquiring acoustic information at multiple distances from the transducer if frequency-dependent attenuation in the target material is a significant factor.
A specific example of these limitations can be seen in in vivo analysis of bone for osteoporosis quantification, where diagnostic techniques have been studied in frequency bands from 300 kHz to greater than 5 MHz and in areas of the body such as the calcaneus (ankle), the hip and lower back. These body regions have varying amounts of muscle and fat tissues between the transducer and bone.
These and other drawbacks are found in known transducers.